In the fields of electric and electronic apparatus, resin materials the volume resistivities of which have been precisely controlled within a semiconductive range of 104 to 1012 Ωcm are demanded. For example, in image forming apparatus (electrophotographic copying machines, electrostatic recording apparatus, etc.) such as copying machines, facsimiles and laser-beam printers of the electrophotographic system, images are formed through steps of charging, exposure, development, transfer, fixing and static charge elimination. In each of these steps, respective members formed of a resin material the volume resistivity of which falls within the semiconductive range are used. In such a semiconductive resin material, the volume resistivity is required to be precisely controlled within a fixed range.
A charging roll or belt, a transfer roll or belt, a conveyer roll or belt, a developing roll, a blade for controlling the thickness of a toner layer, and the like installed in such an image forming apparatus are required to be semiconductive at least at their surface layers, and specifically to have a fixed volume resistivity within a range of 104-1012 Ωcm.
For example, in a charging system making use of the charging roll or belt, voltage is applied to the charging roll, and the charging roll is then brought into contact with a photosensitive drum to directly apply an electric charge to the photosensitive drum so as to equally and uniformly charge the photosensitive drum. In a development system making use of the developing roll, a toner is adsorbed in a charged state on the surface of the developing roll by frictional force between the developing roll and a toner supply roll, and this toner is uniformly leveled by the blade for controlling the layer thickness of the toner, and the toner is then caused to fly toward an electrostatic latent image formed on the photosensitive drum by electric attraction force to develop the latent image. In a transferring system making use of the transfer roll or belt, voltage having a polarity opposite to that of the toner is applied to the transfer roll or belt to generate an electric field, whereby the toner on the photosensitive drum is transferred to a transfer material by electric attraction force generated by the electric field.
Accordingly, the static charge controlling members such as the charging roll or belt in the image forming apparatus are required to have a low volume resistivity within a proper range. The volume resistivity itself is required to be uniform in distribution. If the volume resistivity varies with location, no high-quality image can be provided. For example, if the volume resistivity distribution of the charging roll or belt is uneven, it is difficult to equally and uniformly charge the photosensitive drum, so that the quality of the resulting image is deteriorated. In these members, it is also required that their volume resistivities and surface resistivities do not very vary with the change of environmental moisture. If the volume resistivities and surface resistivities of the static charge controlling members greatly vary with the change of humidity in an ordinary use environment, no high-quality image can be stably provided.
Further, when dust, toner and/or the like are adsorbed on sheathing materials and parts of OA apparatus formed from a resin material, their appearances are impaired, or the cause of trouble is formed. When resin-made apparatus and parts used in the fabrication processes of semiconductor devices, LCD and the like, and films, bags and containers for packaging or conveying electronic parts such as IC and LSI adsorb dust by generation of static electricity, the quality of the electronic parts is impaired. It is therefore required that a volume resistivity of about 104-1012 Ωcm is imparted to resin materials used in these application fields, particularly, in face-layer materials thereof in order for the resin materials to have static charge controlling property.
On the other hand, polyvinylidene fluoride resins are excellent in heat resistance, chemical resistance, stain resistance, anti-adhesion property, molding and processing ability and the like, and thus suitable for use as resin materials for such various static charge controlling members as described above. The polyvinylidene fluoride resins themselves are insulators. As methods for lowering the electrical resistance of the polyvinylidene fluoride resins and molded or formed products (hereinafter may be referred to as “moldings”) thereof, therefore, there have heretofore been proposed (1) a method in which an organic antistatic agent is applied to the surfaces of the resin moldings, (2) a method in which an organic antistatic agent is incorporated into the resins, (3) a method in which a conductive filler such as carbon black or metallic powder is incorporated into the resins, and (4) a method in which an ionic electrolyte is incorporated into the resins.
However, the method (1) involves a problem that since the polyvinylidene fluoride resins have excellent anti-adhesion property, the antistatic agent easily falls off from the surfaces of the moldings by wiping or washing the surfaces. In the method (2), a surfactant or hydrophilic resin is used as the organic antistatic agent. Since the method making use of the surfactant adopts a mechanism that the surfactant is caused to bleed out of the surface of each molding, thereby imparting antistatic property thereto, the volume resistivity and antistatic property of the molding greatly vary with the changes of environments such as temperature and humidity. In addition, high stain resistance, which is a merit of the polyvinylidene fluoride resins, is impaired. In the method making use of the hydrophilic resin, it is necessary to incorporate a great amount of the hydrophilic resin in order to achieve a desired antistatic effect. Therefore, excellent physical properties inherent in the polyvinylidene fluoride resins, such as stain resistance, ozone resistance and solvent resistance, are lowered. In addition, the method involves a problem that the dependency of volume resistivity and antistatic property on humidity becomes high. The stain resistance and solvent resistance are properties required in the case where a toner applied to a member arranged in an image forming apparatus of the electrophotographic system is removed by cleaning. Since an image forming apparatus equipped with a corona discharge device or the like generates ozone, the ozone resistance is also a property required of members in such an apparatus.
The method (3) is adopted in many fields. For example, a roll member such as a charging roll, developing roll or transfer roll is formed by coating a core bar with a semiconductive resin composition obtained by incorporating a conductive filler into a resin. However, the semiconductive resin composition with the conductive filler dispersed in the resin generally has an extremely uneven distribution in volume resistivity. In many cases, a scatter of the distribution amounts to several figures. Therefore, such a semiconductive resin composition has involved a problem from the viewpoint of practical performance. In addition, the charged amount of the conductive filler must be increased in order to attain the required level of semiconductivity. For that reason, there is encountered a problem that the molding and processing ability and mechanical strength of the resulting polyvinylidene fluoride resin composition are lowered, or its hardness becomes too high.
The method (4) of incorporating the ionic electrolyte is expected to be a method effective for imparting semiconductivity to a polyvinylidene fluoride resin, judging from the fact that polyvinylidene fluoride (PVDF) has been known from of old to be a good conductor to ions (for example, Japanese Patent Application Laid-Open Nos. 32330/1976, 110658/1976, 111337/1976 and 127872/1979). However, a resin composition obtained by incorporating an inorganic metal salt such as lithium chloride or potassium chloride, which is a typical electrolyte, into a polyvinylidene fluoride resin has been difficult to lower its volume resistivity to 1×1012 Ωcm or lower, since these inorganic metal salts are only slightly soluble in PVDF. There has also been a problem that aggregate of the inorganic metal salt added in excess forms the cause of fish eye.
When kneading is conducted at a higher temperature or for a longer period of time in order to fully dissolve the inorganic metal salt in the polyvinylidene fluoride resin to prevent formation of the aggregate, the resin and/or the electrolyte is decomposed to impair the mechanical properties and appearance of the resulting moldings. In the case of a deliquescent inorganic metal salt such as a lithium salt, the resulting resin composition becomes hygroscopic when such a metal salt is filled in a great amount. Therefore, this case involves problems that the volume resistivity of the resin composition greatly varies with the change of humidity, and that the surface of the resulting molding becomes sticky due to the deliquescence of the metal salt bled out.
As a method for improving the solubility of an electrolytes in a resin, Japanese Patent Application Laid-Open Nos. 177064/1985 and 72061/1986 have proposed a method in which a polar solvent such as propylene carbonate is contained in the resin. However, this method has involved problems that the Young's modulus of the resin is markedly lowered, and that the surface of the resulting molding becomes sticky due to the electrolyte and solvent bled out.
There have hitherto been proposed methods of using a quaternary ammonium salt as an antistatic agent for resins. For example, Japanese Patent Application Laid-Open No. 64989/1971 discloses an antistatic coating material with a quaternary ammonium salt and a resin dissolved in an organic solvent. However, this coating material is easy to fall off by washing and is hence difficult to retain its antistatic effect for a long period of time.
Japanese Patent Application Laid-Open No. 3835/1972 discloses an antistatic sheet with a quaternary ammonium salt incorporated into a polyolefin. However, this antistatic sheet depends on a mechanism that its antistatic effect is exhibited by bleed-out of the quaternary ammonium salt out of the resin, so that its conductivity and antistatic effect greatly vary with the changes of environments such as temperature and humidity. In addition, since the quaternary ammonium salt is hard to be ionized in a resin having a low polarity, such as the polyolefin, the volume resistivity of the resin cannot be lowered, though its surface resistivity can be lowered.
Further, since most quaternary ammonium salts, particularly, quaternary ammonium halides are poor in heat stability, disadvantages such as foaming and coloring arise in the resulting moldings when such a quaternary ammonium salt is kneaded with a polyvinylidene fluoride resin, the processing temperature of which is as high as 220 to 270° C., to melt-mold the resultant mixture. Even in the case of an ionic electrolyte relatively good in heat resistance, it is necessary to add a great amount of the ionic electrolyte in order to lower the volume resistivity of the polyvinylidene fluoride resin to the desired semiconductive range. However, the use of the polyvinylidene fluoride resin containing a great amount of the ionic electrolyte has involved such problems that mechanical strength of the resulting molding, such as Young's modulus is lowered, that the ionic electrolyte bleeds out to the surface of the resulting molding, that molding and processing are difficult and that cost is increased.
Accordingly, it has been difficult to control the volume resistivity of the polyvinylidene fluoride resin, particularly, to a volume resistivity of 1×1010 Ωcm or lower by only adding the ionic electrolyte. In addition, such an ionic electrolyte as described above has been difficult to stably, uniformly and precisely develop the volume resistivity within a range of 104 to 1012 Ωcm, preferably 105 to 1010 Ωcm and also been hard to make the variation of the volume resistivity by the change of environmental moisture small even when it is used in combination with a conductive filler such as carbon black.
Since many inorganic metal salts and quaternary ammonium salts are easy to bleed out under a high-humidity environment, they have involved problems that metallic impurities on the surface of a static electricity-controlling packaging material used in packaging or conveyance in, for example, a fabrication process of semiconductor devices form the cause of the provision of rejected products, and that in a transfer roll or belt installed in an image forming apparatus of the electrophotographic system, its surface layer undergoes a change in volume resistivity due to the reduction of the ionic electrolyte in the resin, resulting in deterioration of image quality.